Add .github/workflows/nightly.yml for slow quality-insurance jobs #2

Context: Some quality tools (mutants, long fuzzing, loom with high preemption budgets) are too slow for per-PR CI but are high-value when run nightly.

Scope:

• New workflow with cron: '0 4 * * *' and workflow_dispatch.
• Jobs (add stubs even if the underlying tools aren't wired yet):
  • mutants — cargo mutants scoped to leaf crates
  • loom-deep — loom tests with LOOM_MAX_PREEMPTIONS=5
  • fuzz-30m — 30 minutes per fuzz target
  • miri-full — miri over all host-testable crates
• Post a summary comment to an open "nightly tracking" issue on failure (or use a GitHub check-run with annotations).

Acceptance: workflow runs on schedule and reports status somewhere visible.